This invention relates to an electrical power generating plant including a Compressed Air Energy Storage (CAES) System. More particularly, this invention relates to a CAES system which stores excess power from a power grid during off-peak hours, and returns the stored energy to the power grid when required during peak hours. More particularly, this invention relates to a CAES system which uses a piston engine which is operated as an air compressor during off-peak hours to store excess electrical energy from the power grid in the form of compressed air, and is operated as an expander during peak hours to convert the compressed air along with combustible fuels ultimately into electrical energy for the power grid.
The main objective of the electric utility industry is to supply power to the consumer at the lowest possible cost. This has led to the development of large sophisticated nuclear and fossil-fuel-fired steam generating plants. For both technical and economical reasons, these plants should be operated at a steady load. However, to meet daily and seasonal fluctuations in power demand, the industry uses so-called peaker units. The most common form of these units are gas turbine systems which use premium fuels such as natural gas or oil.
Due to our limited supply of natural gas and oil in this country and the current problems in the supply of petroleum fuel from foreign sources, premium fuel has become very expensive and the long-term supply is uncertain. Therefore, electric utilities have been investigating better ways of utilizing or even eliminating the use of premium fuels for peaker units and at the same time operating their large power plants at steady or constant load. This has led to the investigation of energy storage systems.
Studies conducted by electric utilities indicate that CAES power plants are attractive for consideration as an energy storage system. CAES plants which are presently being considered by electric utilities consist of four subsystems: a turbine system; compressor system; air storage reservoir; and a motor/generator. In a CAES plant, off-peak power is used to drive a compressor and the compressed air is subsequently stored in an underground reservoir. Peak power is generated using the stored air together with premium fuel, which is burned in a combustion chamber. In contrast to conventional gas turbine peaker units, the power generating system of a CAES plant is uncoupled from the compressor system so that each system operates independently of the other. The uncoupling of the turbine and compressor system permits the utilization of the full power output of the turbine system to drive the generator. In a conventional gas turbine peaker unit, about one-half to two-thirds of that output is used to operate the compressor. In a CAES plant, therefore, the required capacity (i.e., the gross power output) of the turbine system, as well as the quantity of fuel needed, will be reduced by the same fraction. The required capacity of the compressor system will also be reduced but the amount depends upon the charging and discharging time of the air reservoir.
Various plant configurations are being evaluated by the electric utility industry for air storage pressures in the range of 10-80 atmospheres. Of critical importance to the feasibility of these systems is the total capital cost of plant construction. The storage reservoir is the most costly subsystem of a CAES plant. The size and consequently the cost of the reservoir is proportional to the amount of air needed to burn each pound of fuel supplied. For a CAES plant which uses a turbine-compressor pair, depending upon the inlet gas temperature of the turbine (e.g. 1000.degree.-2000.degree. F.), the air to fuel ratio can vary from 40-60 lbs. of air/lb. of fuel. With the engine described herein, the air to fuel ratio will be reduced to about 18-20 lbs. of air/lb. of fuel. This will be shown to yield a significant economic benefit.